1. Field of the Invention
The present invention relates to a watermark, and more particularly, to a method of and an apparatus for extracting a watermark from repeatedly watermarked data.
2. Description of the Related Art
Due to developments in network-based communications techniques, it has become more frequent to store and transmit multimedia data in a digital format. However, for digital data, it is almost impossible to differentiate original data from copies. Accordingly, predetermined identification information (hereinafter referred to as a “watermark”) needs to be inserted into the original data so that one can identify and/or insist upon one's right to the original data. The predetermined identification information (i.e., the watermark) includes copyright information on the original data, copyrighter's signatures, and other marks.
Generally, a watermark is repeatedly inserted into the original data, and then the repeatedly watermarked data is transmitted. Therefore, even though some of the values of the watermarks inserted into the original data are damaged due to errors occurring during the transmission of the repeatedly watermarked data, data processing, or due to intentional attacks on the original data, it is still possible to extract a complete watermark from the repeatedly watermarked digital data having other intact watermarks. A majority rule manner has been used to select the watermark from among the plurality of watermarks having different values due to errors.
FIGS. 1A and 1B are diagrams illustrating a method of repeatedly inserting a watermark into original data and then transmitting the repeatedly watermarked data. Referring to FIG. 1A, at a sending terminal, a watermark of “101010” is inserted into original data three times so that watermarks 1 through 3 are located at different places of the original data. Thereafter, the watermarked data is transmitted. However, due to attacks against the watermarked data while in a transmission channel, damaged watermarks 1 through 3 are detected at a receiving terminal.
In FIG. 1B, a damaged portion of a watermark is represented with shading lines. In particular, as shown in FIG. 1B, the watermarks 1 and 2 are damaged so that “1010” is deformed into “0101”. Therefore, according to the majority rule method of determining a final watermark, “010110” rather than the original watermark “101010” is determined as the final watermark.
As described above, according to the conventional method of determining a final watermark using the majority rule manner, wrong bit values are more likely to be selected as bit values constituting the final watermark, especially when watermarked data is severely damaged.